The Allen key is an everyday tool that goes by many names. Commonly known as a hex key, Allen wrench, zeta key, or hex head, it is a six-sided tool used as a driver for screws, bolts, and other fasteners designed to fit the tool. The name "Allen wrench" came from the Allen Company, a Hartford, Connecticut based manufacturer who produced the tool. Since then, the tool has gone by many names as numerous manufacturers have produced variations of the wrench.
This tool comes in several different shapes and sizes, but the most typical shape is an L-shape. This allows the user to take advantage of the wrench’s reach — while using the longer arm of the L-shape — or its torque capabilities while using the shorter arm of the L-shape. Some Allen keys come with a ball end on the longer arm of the L-shape to further aid in reach and allowing the wrench to be used at odd angles. The ball end, however, should not be used for torque purposes, as it can easily slip and damage the fastener as well as the tool itself.
Sizes of Allen keys are measured from one flat side to the other as opposed to corner-to-corner. They come in an array of sizes in both metric and standard and are available in extremely small sizes other drivers are not capable of accommodating. One of the advantages of the Allen key is its contact points: six sides in contact with the bolt or screw ensure plenty of surface area to turn the screw effectively.
An Allen key’s versatility becomes apparent when considering the many types of screws and bolts it can accommodate: it works well with recessed screw heads or headless screws as well as high-profile screw heads. Recessed heads and headless screws are protected from external damage and contact, thereby making an Allen key a good choice for exposed screws.
Because these tools are relatively inexpensive to produce, they are becoming more and more common in toolboxes. They are also commonly found included with products that require minor assembly by the consumer and may come in specialty sizes as a result. Bicycles, exercise equipment, furniture, and some home electronics commonly come with these tools included. More and more Allen keys are included in drill bit sets as well, and individual bits are available at most hardware stores.